By: Jussi Heikkinen, Wärtsilä, Ray Baumgartner, Siemens & Andy Baxter, GE Energy, USA
The dilemma of steam cycle power plants that are typically fired with coal or natural gas (combined cycles) is that the steam system takes several hours to start up. Such plants are not flexible regarding starting and loading. They prefer steady high load operating conditions and reward that with good efficiency and reliability.
Furthermore, the inevitable impact of increasing wind power capacity in the grids is that baseload plants will get fewer hours of full power and increasingly need to regulate and follow the load that is left after windpower plants have taken their continuously varying share. To a certain extent, steam cycle power plants can be used to follow changes in windpower generation, but possibilities to use such power plants as fast starting capacity reserve or to deliver grid stability services are very limited. These plants are good for a single function: base load generation.
Because peaking and reserve capacity is needed only during the high consumption periods of the year, the energy-based electricity markets do not provide adequate revenues to justify investments in such power plants. The traditional utility approach to providing capacity reserves used to be to install large 200 MW industrial gas turbines close to large cities. Due to their characteristics, these turbines are seldom useful for anything other than pure capacity reserves in extreme contingency conditions. First, such turbines offer poor fuel efficiency, leading to high generating costs. Second, the startup time of such a turbine from start signal to full load has typically been around 25 minutes, which is not short enough to allow them to function as first or secondary reserves, or to deliver non-spinning reserve in the USA. Frequent starting and stopping also increase the maintenance costs of such turbines. Thus for these reasons, these plants are good for a single function only.
In free markets, single-function power plants will not always offer the flexibility that future customers are going to be looking for. We are all familiar with stranded assets, or those power plants that, due to changed market conditions, became somehow obsolete and incurred economic losses. A growing portion of the generation capacity needs to be able to act in multiple functions, providing competitive baseload generation cost, grid stability services, fast load to match rapidly increasing windpower capacity and, on top of that, low emissions to meet the norms of today and the future. It is still a surprise to many people that modern high-tech reciprocating engine power plants can provide all of these.
Instead of single-function power plants, customers increasingly need to look for power plant alternatives that could take care of all of the following tasks, not just one of them: covering the baseload à‚— or intermediate load à‚— generation segment with a good heat rate, in other words, high efficiency; possessing and providing dynamic reserves for grid stability; and ensuring adequate dynamic load-following capacity for rapidly increasing wind generation.
Startup time increasingly valuable
Let us, as an example, look at what benefits a short starting time could provide the plant owner. By startup time we mean the time it takes from the start order for a power plant to: safety check and purge its systems before starting to rotate the equipment; accelerate the shaft to synchronous speed; synchronize with the grid; and ramp up the load from zero to 100 per cent load.
Barrick’s on-site power plant in Nevada, USA, where Wärtsilä fast-startup gas engines has a net output 116 MW
It is obvious that the faster the starting and loading sequence the better. This provides certain advantages. For example, when operating against a power tariff, only certain tariff hours are profitable to operate on full power. In some regions, starting and stopping is necessary several times a day, so the ability to start quickly increases revenues and/or reduces the negative ones. Also, startup emissions are always an issue. The faster the sequence, the lower the emissions.
When a plant operates as a non-spinning reserve, a faster starting time is always an advantage. Ten minutes has often been the norm, and the fact is that there are not many technologies that can really start and achieve full load within that time.
Reciprocating engine power plants have typically had a starting time of eight to ten minutes, depending on the design of the starting air system and the level of engine preheating. The engines are capable of starting and loading much faster, but because this was not required by the markets in the past, shorter starting times were not provided by the manufacturers.
As requirements for fast starting have emerged recently in many markets, Wärtsilä has changed the design of its power plants. It now offers its gas power plant customers the option of an ultra-short, five-minute startup time. This requires certain changes in the plant design. The starting air system has to be dimensioned for simultaneous starting of all units; all units have to have separate synchronization software to enable simultaneous synchronization with the grid; the engines need to be kept in hot standby (80 à‚°C) instead of warm standby (40 à‚°C); the preheating system must be modified; and, if hot standby is often needed, Wärtsilä recommends the use of a hot water boiler fired by natural gas.
A total plant solution
Also offering lower emissions and higher efficiency during startups is GE Energy. It has introduced a ten-minute start capability for its Frame 7FA gas turbines as an expansion of its OpFlex gas turbine technology programme, which provides operational flexibility enhancements for gas turbines. The 7FA with the new startup capability will be able to dispatch in ten minutes after a start signal and will achieve stable combustion capable of nitrogen oxides (NOX) and carbon moNOXide (CO) emissions of 9 parts per million (ppm) within that time. When integrated into GE’s next generation Rapid Response combined-cycle power plant design, the turbine has the ability to reduce startup emissions for a 207FA system (two gas turbines and one steam turbine) by as much as 20 per cent and increase starting efficiency by up to 30 per cent.
“A power company using GE’s Rapid Response combined-cycle power plant design with ten-minute start capability can provide high-efficiency power when it is needed most,” says John Reinker, general manager of the heavy-duty and combined-cycle gas turbine product line for GE Energy. “It is designed for customers who want to extend operation under an emissions cap, are contemplating cyclic duty or have an opportunity to tap into additional revenue from the ancillary market.”
The 7FA with ten-minute start capability employs advanced fuel scheduling and purge credit technologies, complementing GE’s Rapid Response power plant technology to create a total plant solution that features gas and steam turbine, generator and HRSG engineering, designed for flexible and responsive operation. It targets 60 Hz markets and will be available for simple-cycle applications in 2009 and combined-cycle operation in late 2010.
Flexing a plant’s muscles
When electric power producers asked how the need for cost-competitive peak and intermediate power generation would be met while maintaining environmental friendliness, Siemens replied with the SCC6-5000F 1×1 Flex-Plant 10, a unique solution that also answered the critical need for qualified ten-minute, non-spinning reserve.
The SGT6-5000F with the fast-start package is the only industrial-frame, gas turbine generator set that has been demonstrated to generate 150 MW in as little as ten minutes as well as full load capabilities in about 12 minutes, all without imposing additional start-related maintenance impacts. With a high, F-class, simple-cycle net efficiency of over 38 per cent, the package is well suited to answer the need for peak load applications. However, as the demand for peak to intermediate operation has grown from 500 hours to 2000-3000 hours per year, simple-cycle efficiency at today’s gas prices becomes cost prohibitive. At the same time, the high capital costs and complexities of traditional combined-cycle plants make them less suitable for this operational mode.
While working with key customers, Siemens looked to its broad fast-start technology portfolio to develop a uniquely integrated plant solution that synergized the ten-minute start capability of its SGT6-5000F gas turbine and its Benson evaporator technology to provide a solution-optimized product.
The key product requirements were set as follows: retain the ten-minute gas turbine start capability; keep emissions very low using proven technologies; keep water consumption very low; and maximize plant efficiency with minimized plant complexity. The result is the SCC6-5000F 1×1 Flex-Plant 10, a 275 MW plant that meets these requirements by linking Siemens’ fast-start gas turbine to a state-of-the-art, single-pressure, air-cooled bottoming cycle with optimized steam temperature and pressure of circa 499 à‚°C and 1500 psia.
To capitalize on their knowledge and expertise with the Benson once-through HRSG technology, Siemens worked closely with key HRSG suppliers in the development of a modified drum-type evaporator that is specifically designed to handle fast starting at the optimized steam operating pressures and temperatures used in the Flex-Plant 10.
Extensive HRSG transient analysis work was also conducted as part of the overall lifetime analysis programme that validated the design. The HRSG is designed with an industry-proven conventional selective catalytic reduction (SCR) system, which together with a standard CO catalyst reduces plant emissions to best available control technology levels. The one-casing SST800 steam turbine is of the single-pressure, non-reheat, non-condensing design and ships fully mounted on a bedplate to the jobsite for simplified installation. The air-cooled generator is also shipped to jobsite fully assembled. To minimize water use, an air-cooled heat exchanger condenser is used.
After an overnight shutdown, a typical startup of the Flex-Plant 10 shows the advantages of the integrated plant design. Enabled by a Siemens static frequency converter, the generator is used to assist the gas turbine fast ramp to synchronization, and does it without the need for overfiring the turbine. Durations from the initiation of the start sequence to the generation of 150 MW with NOX emissions of 9 ppm dry can be as little as ten minutes. With the installation of standard SCR system and CO catalysts, stack emissions compliance is reached in about 20 minutes. Steam that is produced by the HRSG is bypassed to the condenser until the steam turbine is ready to accept the steam flow. Full combined-cycle plant load is achievable in about 45 minutes with an average startup efficiency of almost 40 per cent.
Further development is being conducted within Siemens to integrate additional fast-start features from its Flex-Plant 30 series combined cycles to further reduce startup times. Not only will this increase the amount and timeliness of saleable power, but it will also further improve startup efficiency and startup emissions.
Utilization of the fast-start gas turbine has additional advantages regarding CO, NOX and volatile organic compound (VOC) emissions. While the fast-start technology used in the Flex-Plant 30 already greatly reduces the startup emissions from a traditional combined-cycle triple-pressure reheat plant, the Flex-Plant 10 cuts these emissions by a nominal 86 per cent on a warm-start basis. A further feature of the Flex-Plant 10 is that by adding evaporative cooling and steam power augmentation, the plant power output and heat rate remain stable over the entire ambient temperature design range. This is summarized in Figures 1 and 2.
Figure 1: Evaporative cooling and steam power augmentation keep the net power output (MW) of the Siemens Flex Plant 10 stable with changes in ambient temperature
Figure 2: Heat rate (Btu/kWh) of the Siemens Flex Plant 10 remains stable over the whole ambient temperature range